def line_stepper(start, k):
    """
    Complete the function line_stepper, which returns the number of ways there are to go from
    start to 0 on the number line by taking exactly k steps along the number line.

    >>> line_stepper(1, 1)
    1
    >>> line_stepper(0, 2)
    2
    >>> line_stepper(-3, 3)
    1
    >>> line_stepper(3, 5)
    5
    """
    if start == 0 and k == 0:
        return 1
    elif k == 0:
        return 0

    left = line_stepper(start - 1, k - 1)
    right = line_stepper(start + 1, k - 1)

    return left + right

